3-Hydroxybenzaldehyde (1 g, 10 mmol) was used as starting material and dissolved in acetonitrile (50 mL). To this solution, p-toluenesulfonic acid was added in batches at room temperature and after stirring for 5 minutes, N-chlorosuccinimide (NCS, 1.33 g, 10 mmol) was added. The reaction mixture was continued to be stirred at room temperature for 2 hours. Upon completion of the reaction, the reaction was quenched with aqueous sodium thiosulfate and subsequently diluted with ethyl acetate and brine. The organic layer was separated, dried with anhydrous sodium sulfate and concentrated under reduced pressure to give the crude product. The crude product was purified by silica gel column chromatography with petroleum ether/ethyl acetate (10:1 to 5:1) as eluent to afford 2-chloro-5-hydroxybenzaldehyde (340 mg, 21.7% yield, Intermediate 28a) and 4-chloro-3-hydroxybenzaldehyde (310 mg, 19.7% yield, Intermediate 28b) as yellow solids, respectively. 2-chloro-5-hydroxybenzaldehyde's 1H-NMR (CDCl3) data: δ 10.43 (s, 1H), 7.54-7.56 (d, 1H), 7.30-7.37 (m, 2H). 4-Chloro-3-hydroxybenzaldehyde 1H-NMR (CDCl3) data: δ 10.43 (s, 1H), 7.40 (d, 1H), 7.33 (d, 1H), 7.06- 7.09 (dd, 1H).
